The acetyl CoA: acetoacetate CoA transferase of Escherichia coli catalyzes the transport of acetoacetate and other short chain monocarboxylic acids into E. coli. Studies are proposed to determine the primary sequence of a peptide that contains the active center of the enzyme. The active center was specifically radiolabeled by reduction of the covalent enzyme-CoA intermediate with NaB 3H4. Studies are also underway whose aim is to examine the environments of sulfhydryl groups in the enzyme. The environments of these sulfhydryl groups are altered during several conformational changes in the catalytic pathway. The experimental approach uses optical and paramagnetic reporter groups. Steady state and transient kinetic studies on the general acyl CoA dehydrogenase are also proposed. These studies are preliminary to examining the relationships between acylcarnitine transport, acyl CoA formation in the mitochondrial matrix and beta-oxidation of fatty acyl CoA's.